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© 2017 Ebneshhidi
Functions of the urinary system
Excretion – removal of waste material from the blood plasma and
the disposal of this waste in the urine.
Elimination – removal of waste from other organ systems. From
digestive system – undigested food, water, salt, ions and drugs.
From respiratory system – CO2 , H+, water, and toxins. From skin –
water, NaCl, and nitrogenous wastes (urea, uric acid, ammonia,
creatinine).
water balance – kidney tubules regulate water reassertion and
urine concentration.
Regulation of pH – volume, and composition of body fluids.
production of erythropoietin – for hematopoieses, and renin
for blood pressure regulation.
© 2017 Ebneshhidi
Anatomy of the urinary System
kidneys – a pair of bean – shaped organs located
retroperitoneally, responsible for blood filtering and urine
formation.
Renal capsule – a layer of fibrous connective tissue covering
the kidneys.
Renal cortex – outer region of the kidneys where most
nephrons are located.
Renal medulla – inner region of the kidneys where some
nephrons are located, also where urine is collected to be
excreted outward.
Renal calyx – duct – like sections of renal medulla for
collecting urine from nephrons and direct urine into renal
pelvis.
© 2017 Ebneshhidi
Renal pyramid – connective tissues in the renal medulla binding
various structures together.
Renal pelvis – central urine collecting area of renal medulla.
Hilum – concave notch of kidneys where renal artery, renal vein,
ureter, nerves, and lymphatic vessels converge.
Ureter – a tubule that transport urine (mainly by peristalsis) from
the kidney to the urinary bladder.
Urinary bladder – a spherical storage organ that contains up to
400 ml of urine.
Urethra – a tubule that excretes urine out of the urinary bladder to
the outside, through the urethral orifice.
© 2017 Ebneshhidi
Microscopic Anatomy
Each kidney consists of about 1 million basic functional units
called nephrons where blood filtering and urine formation occur.
Each nephron is composed of the following parts:
afferent arteriole → glomerulus → bowman's capsule →
efferent arteriole → proximal convoluted tubule (PCT) →
descending limb of loop of henle → loop of henle →
ascending limb of loop of henle → distal convoluted
tubule(DCT) → collecting duct (not part of the nephron).
molecules in the blood that will be transformed to become part of
urine travel through the above structures, while molecules that will
be retained and reabsorbed back to the blood will come out of the
bowman's capsule, and go into efferent arteriole and the
peritubular capillaries.
© 2017 Ebneshhidi
Urine Formation
urine formation involves 4 processes:
Filtration – small molecules are filtered from glomerulus's
to bowman's capsule.
Rebasorption – nutrient molecules are transported from
PCT and DCT to per tubular capillaries.
Concentration – water is reabsorbed from descending limb
of loop of handle and from collecting duct into peritubular
capillaries.
Secretion – waste or harmful substances are transported
from peritubular capillaries to PCT and DCT.
© 2017 Ebneshhidi
Glomerular Filtration
Small molecules in blood
plasma are forced from the
glomerulus to bowman's
capsule, through the pores in
the capillary walls of
glomerulus.
Any molecules smaller than
the plasma proteins will be
filtered across – e.g. water,
glucose, amino acids, fatty
acids, vitamins, minerals,
electrolytes, antibodies,
enzymes, hormones, drugs,
and nitrogenous wastes.
© 2017 Ebneshhidi
Functions of nephron components
Renal capsule:
Glomerulus: filtration of H2O and dissolved substances from the
plasma.
Glomerular capsule: receives the glomerular filtrate.
proximal convoluted tubule:
Reabsorption of glucose, amino acids, creatine, lactic acid, citric, uric,
and ascorbic acids; phosphate, sulfate, calcium, K, and Na by active
transport.
Reabsorption of proteins by pinocytosis. Reabsorption of H2O by
osmosis. Reabsorption of Cl- and other negatively charged ions by
electrochemical attraction.
Active secretion of substances such as penicillin, and hydrogen ions.
© 2017 Ebneshhidi
Descending limb of nephron loop:
Reabsorption of H2O by osmosis.
Ascending limb of nephron loop:
Reabsorption of Na, K, and Cl- by active transport.
Distal convoluted tubule:
Reabsorption of Na by active Transport.
Reabsorption of H2O by osmosis.
Active secretion of hydrogen ions.
Secretion of K both actively and by electorchemical attraction
(passives).
Collecting duct:
Reabsorption of H2O by osmosis.
© 2017 Ebneshhidi
Juxtaglomerular Apparatus
The JG apparatus is located at the point of contact between the
distal convoluted tubule and the afferent and efferent arterioles.
In its convolutions, the DCT comes into very close contact
with the afferent arterioles.
At this point the cells in the afferent arteriols are more
numerous, forming a cuff, and are called JG cells these are
mechanoreceptors that detect changes in Blood pressure in
the afferent arterioles, and secrete renin.
The distal convoluted tubule cells contacting these JG cells
are called macula densa (chemo or osmoreceptors) that
respond to changes in the solute concentration of the filtrate,
in the tubule.
© 2017 Ebneshhidi
Vasa recta:
capillaries of the
juxtamedullary
nephrons that loops
and have a hairpin
configuration,
forming a bundle of
long straight vessels.
© 2017 Ebneshhidi
Glomerular Filtration
a. urine formation begins when waste and water and dissolved
materials are filtered out of the glomerular capillary.
Urinary excretion = glomcrular filtration + Tubular secretion –
tubular reabsorption
b. the glomerular capillaries are much more permeable than the
capillaries in other tissues.
Filtration pressure = forces favoring filtration (Glomerular capillary
hydrostatic pressure & capsular osmotic pressure) – forces
opposing filtration (capsular hydrostatic pressure & Glomerular
capillary osmotic pressure).
Thus, filtration pressure is the net force acting to move material out
of glomerulus and into the glomerular capsule.
© 2017 Ebneshhidi
Regulation of GFR
Neural regulation where sympathetic nerves, upon the activation of
chloride ion levels, can cause the constriction or relaxation of the
afferent arteriole, resulting in a change of GFR.
Renal autoregulation where the juxtaglomerular apparatus
(JGA) (formed by the afferent arteriole and DCT) secretes
vasoconstriction substances to either afferent arteriole, in response
to GFR changes and NaCl levels.
Hormonal regulation involves the JGA secreting a hormone called
renin which activates an inactive hormone from the liver called
anigotensinogen , resulting in an active hormone (angiotenesin I)
which will be converted to angiotensin II by the angiotensin
converting enzyme (ACE) [released from the lungs]. Angiotensin II
causes constriction of afferent arteriole & release of Aldosterone
from adrenal cortex which leads to salt and water retention.
© 2017 Ebneshhidi
GFR
The rate of filtration varies with filtration pressure. Filtration
pressure changes with the diameters of the afferent and efferent
arterioles.
Constriction of afferent arterioles due to sympathetic stimulation
decreases glomerular filtration rate.
As the osmotic pressure in the glomerulus increases, filtration
decreases.
As the hydrostatic pressure in a glomerular capsule increases,
filtration increases.
The kidney produce 125 ml of glomerular fluid per minute, most of
which is reabsorbed.
The volume of filtrate varies with the surface area of the glomerular
capillary.
© 2017 Ebneshhidi
Regulation of Filtration Rate
a. Glomerular filtration rate remains relatively
constant by may increase or decrease when
needed. Increased sympathetic activity decreases
GFR .
b. when tubular fluid Nacl decreases, the macula
densa causes the JG cells to release renin which
leads to vasoconstriction, which affect GFR, and
secretion of aldosteron, which stimulate tubular
Na+ reabsorption.
© 2017 Ebneshhidi
Role of ADH ( Antidiuretic hormone)
1. Concentration of H2O in the blood decreases.
2. Increase in the osmotic pressure of body fluids stimulates
osmoreceptors in the hypothalamus.
3. Hypothalamus signals the post. pituitary gland to release
ADH.
4. Blood carries ADH to the kidneys.
5. ADH causes the distal convoluted tubules and collecting
ducts to become more permeable and increase H2O reabsorption
by osmosis.
6. urine becomes more concentrated, and urine volume
decreases.
© 2017 Ebneshhidi
Mechanisms of Urine Formation
Urine formation and adjustment of blood composition involves three major processes
Glomerular filtration
Tubular reabsorption
Secretion
© 2017 Ebneshhidi
Tubular Reabsorption
The kidney must have mechanisms for reabsorption of the many
solutes (Na+, K+, glucose, chloride -) and H2O that it filters each
day or in a matter of minutes we would by depleted of all these
substances.
Substances are selectively reabsorbed from the glomerular filtrate.
The preritubular capillary is adapted for reabsorption. It carries low
pressure blood & is very permeable. Most reabsorption (70%),
occurs in the proximal tubule.
Different modes of transport reabsorbs various substances in
particular segments of renal tubule.
Glucose and amino acids by active transport. H2O is reabsorbed by
osmosis. proteins are reabsorbed by pinocytosis.
© 2017 Ebneshhidi
Water Reabsorption (Proximal Tubule)
Na+ and K+ ions are
reabsorbed by active
transport.
Negatively charged ions are
attracted to positively
charged ions (passive
transport).
As the concentration of ions
(solute) increases in plasma,
osmotic pressure increases.
Water (70%) moves from
renal tubule to capillary by
osmosis (passive transport).
© 2017 Ebneshhidi
Countercurrent Mechanism
The fluid entering the loop of Henle has an osmolarity of 100 mosm/l.
2) Thus, a small horizontal gradient of 200 is established between the
ascending and descending limbs.
3) This occurs because of the characteristics of each portion of the
loop :
The descending limb is very permeable to H2O (out) and
to Na+ and Cl- (in). [Cl- follows Na+ because of electrical
attraction].
No Active transport of ions occurs here.
The Ascending limb is impermeable to H2O but actively
transports Cl- out of the tubular fluid into interstitial fluid,
with Na+ ion following passively.
© 2017 Ebneshhidi
Thus, this small horizontal osmolar gradient is due to active
pumping of salt out of a H2O impermeable ascending limb into
both the ISF and the descending limb, and H2O movement out of
the descending limb.
© 2017 Ebneshhidi
Summary of events in the loop of henle
A. Fluid enters the descending
limb of the loop. At each
horizontal level Cl- is actively
transported out of the
ascending limb into the ISF.
Na+ follows & diffuses out of
the ascending limb into the
ISF. H2O can not leave the
ascending limb. Thus, the
osmolarity of fluid in the
ascending limb decreses as
you go up and within the ISF.
It increases as you go deeper
in the medulla.
© 2017 Ebneshhidi
The descending limb is
permeable to H2O. Water
moves passively out of the
descending limb into the
ISF.
This causes the conc. of
Nacl in the descending limb
to increase, and this fluid
also increases in osmolarity.
The net overall result is that
an osmolar gradient is
established in the ISF as one
progresses from the
beginning to the end in the
loop of Henle.
© 2017 Ebneshhidi
Thus, more fluid has been reabsorbed from the original volume of
glomerular filtrate.
The loop has actually placed more solute than H2O into the medullary
interstitial space and so as fluid leaves the loop and enter the DCT it is
hypoosmotic to plasma.
© 2017 Ebneshhidi
The distal convoluted tubule and collecting duct are impermeable
to H2O, so water may be excreted as dilute urine.
If ADH is present, these segments become permeable, and water
is reabsorbed by osmosis into the hypertonic medullary interstitial
fluid.
ADH stimulates H2O reabsorption and the production of
concentrated urine which contains soluble waste and other
substances in a minimum of H2O, thus minimizing the loss of
body H2O when dehydration is a threat. If the body fluids contain
excess H2O, ADH secretion is decreased and the DCT and CD
becomes less permeable to H2O.
Aldosterone secreted by adrenal cortex causes more sodium
reabsorption at DCT, and the positive charges of these ions attract
water molecules to be reabsorbed also at DCT.
© 2017 Ebneshhidi
Tubular secretion
Unwanted substances such as wastes and excessive
salts are secreted by the peritubular capillaries to
the renal tubules (mainly PCT and DCT), so that it
can be disposed in the urine.
Most substances are secreted by active transport.
Substances secreted include excessive Na+, Cl-, H+,
K+, histamine , creatinine, ammonia, uric acid,
vitamins, and excessive drugs.
© 2017 Ebneshhidi
Major events of micturition
urinary bladder distends as it fills with urine.
stretch receptors in the bladder wall are stimulated and
signal the micturition center in the sacral spinal cord.
parasympathetic nerve impulses travel to the detrusor
muscle, which respond by contracting rhythmically.
The need to urinate is sensed as urgent.
voluntary contraction of the external urethral sphincter and
inhibition of the micturition reflex by impulses from the
brain stem and the cerebral cortex prevent urination.
following the decision to urinate, the external urethral
sphincter is relaxed, the impulses from the pons and
hypothalamus facilitate the micturation reflex.
© 2017 Ebneshhidi
The detrusor muscle contracts and urine is expelled through the
urethra.
Neurons of the micturition reflex center fatigue, the detrusor
muscle relaxes, and the bladder begins to fill with urine again.
© 2017 Ebneshhidi
Physical properties of urine
Transparency is clear, indicating the lack of large solutes such
as plasma proteins or blood cells. [can be influenced by bacterial
metabolism in older urine samples].
Color is from light yellow to amber, due to urochrome pigments
as byproduct of bile metabolism [can be influenced by food,
menstrual bleeding, and minor metabolic products].
Odor is from aromatic to slightly ammonia – like, due to the
nitrogenous wastes in urine [can be influenced by disorders such
as glycosuria where urine shows a sweet odor, or by food such as
garlic, or by drug].
pH is from 4.6 to 8.0 with an average of 6.0, due to H+ in the
urine [strongly influenced by diet where protein cause acidic
urine, and vegetables and wheat cause alkaline urine].
© 2017 Ebneshhidi
Specific gravity (a measurement of dissolved solutes in a
solution) is from 1.001 to 1.035, due to the 5% solute
composition in normal urine.
Volume is 1-2 liters per day (about 1% of filtration input).
[can be influenced by body activities, water intake, hormonal
regulation, or disorders such as diabetes insipidus].
© 2017 Ebneshhidi
Abnormal Constituents Of Urine
Albumin – a large plasma protein that should not be filtered out
of glomerulus; when it is present, it is called albuminuria which
may be due to kidney infection called glomerulonephritis.
Glucose – a nutrient molecules that should have been reabsorbed
(in the case of high carbohydrate diets, trace amount of glucose
may be found in urine); when is present, it is called glycosuria
which may be due to insulin – related problems in a disease
called diabetes mellitus.
blood or erythrocytes – any blood cell should not be filtered out
of glomerulus or be present in the urine (except in menstruation –
related bleeding); when it is present, it is called Hematuria
which may be caused by glomerulonephritis, hemolytic anemia,
or urinary tract in infections.
© 2017 Ebneshhidi
Hemoglobin – pigment protein that normally should be enclosed
in erythrocytes and not filtered out of glomerulus; when present, it
is called hemoglobinuria which may indicated hemolytic anemia.
Leukocytes – large white blood cells that should not be present in
urine (except in UTI where leukocytes are present to fight the
infection); when it is present, it is called Pyuria which may be
caused by glomerulus's nephritis, UTI, or even strenuous exercise.
Ketones – by product of metabolism that may occur in trace
amounts, but not large quantities in the urine; when it is present, it
is called Kentonuria which may indicate certain infections in the
urinary system.
Bilirubin – a bile pigment that is normally recycled in lipid
metabolism; when it is present, it is called bilirubinuria which
may be due to abnormal lipid metabolism, or certain infections in
the urinary system.
© 2017 Ebneshhidi
Clinical Terms
Bacteriuria: Bacteria in urine.
Diuresis: increased production of urine.
Diuretic: substances that increase urine production.
Dysuria: painful or difficult urination.
Hematuria: Blood in urine.
Polyuria: excess urine.
Uremia: urine in blood.
Glomerulonephritis: Inflammation of glomeruli, damaging the
filtration membrane, increasing its permeability (may be due to
streptococcal bacteria).
Urinalysis: Analysis of urine to diagnose health or disease.